Bone Anchored Hearing Device (BAHD / BAHA)

Overview and Interface with the Auditory System

Rationale and Patient Population

• Chronically draining ears (chronic suppurative otitis media)
• Mastoid bowl, meatoplasty, or surgically closed external auditory canals
• Aural atresia, canal stenosis, or microtia
• Otosclerosis or tympanosclerosis with contraindication to surgery
• Intolerance of ear mold occlusion
• Unilateral sensorineural hearing loss / single-sided deafness (SSD)

Historical Background

• Per-Ingvar Brånemark (Gothenburg, Sweden, 1960s) discovered that titanium could become firmly anchored in bone without interposed soft tissue — he coined the term "osseointegration."
• Osseointegration was initially applied to dental implants. By 1975, Brånemark's group began studying percutaneous skin-penetrating implants.
• Tjellström and colleagues first coupled a bone vibrator to an osseointegrated implant in 1977 at the Institute of Applied Biotechnology in Sweden.
• By the 1980s, a clinical program was established providing titanium percutaneous implants for facial prosthetics and bone-anchored hearing devices.

Principle of Osseointegration

Implant Material

• Pure titanium: 99.75% pure
• Titanium alloy: 90% Ti, 6% Al, 4% V

Key Biophysical Requirements for Successful Osseointegration

1. Implant surface chemistry — titanium oxide layer forms stable direct contact with bone
2. Implant surface geometry — threaded design maximises surface area
3. Surgical technique — atraumatic, low-speed drilling with copious irrigation to prevent thermal damage
   • Osteocytes are lost if tissue is heated to ≥47°C for 1 minute
   • High-speed drilling can generate temperatures as high as 89°C — irrigation is essential
4. Biomechanical loading — optimal force transfer without micromotion at the interface
5. Host bone quality — adequate cortical bone thickness (minimum ~3 mm in adults; skull may be thinner in children <5 years)

Implant Design

The BAHA System — Components

Indications

Conductive and Mixed Hearing Loss

• Pure-tone average (PTA) bone conduction threshold <45 dB HL = best results (89% improvement rate)
• PTA 45–60 dB HL: still beneficial (61% improvement)
• PTA >60 dB HL: limited benefit (22% improvement) — poor cochlear reserve limits what bone conduction can achieve
• Minimum speech discrimination score on the better ear should be adequate for meaningful benefit

Single-Sided Deafness (SSD) / Unilateral SNHL

• The BAHA acts as a contralateral routing of signal via bone conduction (CROS via bone)
• Sound picked up on the deaf side is transmitted across the skull to stimulate the better cochlea
• Studies show significantly better speech recognition in noise with BAHA vs. CROS hearing aids
• Sound localization does NOT improve (only one cochlea is stimulated)
• Importantly, BAHA reduces informational masking — a study by May et al. showed that adding OBCH to SSD patients reduced error rate in competing sentence conditions from 40% to 24%
• Lin et al. found consistent satisfaction with Baha amplification and poor acceptance of CROS amplification in unilateral SNHL

Surgical Technique

Single-Stage Procedure

Traditional Technique

1. Site selection: Posterior to the auricle, typically 5–5.5 cm from the external auditory canal
2. Skin preparation: Hair-bearing skin is thinned and hair follicles removed. The final skin flap should be <1 mm thick at the implant site. A Cochlear-specific dermatome minimizes skin necrosis
3. Bony preparation: Cortical bone drilled with a slow high-torque drill (1500–3000 rpm) using sharp, disposable bits with copious irrigation
4. Implant insertion: Self-tapping titanium screw inserted; abutment attached
5. Skin closure: Edges sutured to periosteum to minimize tension

Minimally Invasive Techniques (Modern)

• Linear skin incision without skin thinning, or a single skin punch
• Abutment must protrude ≥3 mm above the skin surface
• Multiple studies show no increase in skin complications vs. traditional techniques; some show lower complication rates
• Advantages: significantly decreased surgical time, decreased healing time, improved cost-effectiveness

Pediatric Considerations

• Children younger than 5 years of age may have inadequate skull thickness for standard implant
• A soft-band headband is used in young children as a non-surgical interim solution, allowing early auditory stimulation while awaiting adequate skull development
• Once older, children proceed to surgical implantation

Outcomes and Results

Large Series (Håkansson et al., 10-year experience, n=147):

• Speech discrimination scores improved from 14% unaided → 81% with BAHA (89% if PTA <45 dB)
• Recommended PTA threshold for "high success rate": <45 dB HL (improvement still expected up to 60 dB)

U.S. Experience (initial review):

• Common indications: chronic otitis media/draining ears, EAC stenosis/aural atresia, post-skull base surgery canal closure
• Average functional gain: ~25 dB across speech frequencies

Children (Birmingham experience):

• 4 of 7 children showed ≥5 dB PTA improvement over their prior aid
• Average speech discrimination ~90% (vs. 0% unaided)

Complications

Skin/Soft Tissue Complications (Holgers Classification)

• Skin reactions (Holgers Grade 2–4): 2.4–38.1%
• Failure of osseointegration: 0–18%
• Revision surgery (adults): 1.7–34.5%
• Revision surgery (pediatric): 0–44.4%

• Wound infection, hematoma
• Implant extrusion
• Numbness/dysesthesia around implant site

Alternatives: Transcutaneous Bone-Anchored Hearing Devices

1. Alpha 2 (Sophono Inc.)

• First used in Germany 2006; FDA clearance 2011
• External component: sound processor + bone-conducting oscillator
• Implanted component: titanium-encased samarium-cobalt twin magnetic implant, secured with 5 titanium screws into a bony recess (≥2.6 mm deep)
• Magnetic coupling transmits vibrations transcutaneously — no percutaneous abutment
• Skin flap thinned to 4–6 mm in adults
• FDA conditional MRI approval up to 3T; head shadow ~5 cm
• Indications: age >5 years; CHL/MHL with conductive component ≤45 dB; SSD with BC ≤20 dB

2. BAHA Attract (Cochlear)

• FDA clearance 2013
• Magnet-based transcutaneous system
• External magnet-coupled processor; implanted magnet

3. Bonebridge (MED-EL)

• CE mark (Europe); FDA approval July 2018
• Unique active transcutaneous design: the transducer itself is implanted subcutaneously
• Relatively newer data in the literature

Summary Comparison